ORAL SESSION 4E5: Charge Ordering

Thursday, Feb. 24, 10:00 a.m. – 12:15 p.m., Room 301D (GRB)

Chair: V. Emery (Brookhaven Nat'l. Lab)

4E5.1 Stripe ordering and two-gap model for underdoped cuprates

C. Castellani, C. Di Castro, M. Grilli, and A. Perali, Dipartimento di Fisica, Universita' La Sapienza, Roma, Italy

Presenting Author: C. Castellani

The evidence of edge-gaps around the M-points well above the superconducting critical temperature T_c in underdoped cuprates has triggered a very active debate on their origin. We first consider the possiblity that this spectroscopic feature results from a well formed quasi-static CDW ordering taking place in the underdoped regime. To explain the coexistence of gaps and arcs on the Fermi surface we find that the charge modulation should be in an eggbox form (2D regular lattice of 1D domain walls). In the lack of evidence of such a peculiar CDW we then investigate the local pairing induced by dynamical stripe fluctuations. We find that a proper description of the strong anisotropy of both the interactions and the Fermi velocities requires a two-gap model of pairing. By evaluating the superconducting fluctuation propagators we find that the gap near the M-points experiences strong short wavelength fluctuations below a mean field pairing temperature (where the M-gap first forms) until phase coherence is established by coupling to the stiffness of the pairing near the nodal points. By tuning the anistoropic interaction the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimal doped regime) to a mixed boson-fermion model (in the strongly underdoped regime).

4E5.2 The Study of the Stripe Phase La_1.48Nd_0.4Sr_0.12CuO₄ by electrochemical doping

Pei-Herng Hor, Zu-Gang Li, Bernd Lorenz and Young-Seok Song, Department of Physics and Texas Center for Superconductivity at the University of Houston, Houston, TX 77004-5932

Presenting Author: P.-H. Hor

We have studied the effects of hole doping into the stripe phase composition La_1.48Nd_0.4Sr_0.12CuO₄. Precise amount of holes, in steops of increasing hole concentration D p=0.002-0.003, were introduced into the sample by electrochemical oxidation. At room temperature, we have observed an anomalous jump of thermoelectric-power at p=0.125± 0.001. The LTO to LTT structural transition temperature (T_o) decreases with increasing p and disappears above p=0.13. In contrast, T_c increases quickly for p>0.13 and saturates at T_c=30K for p>0.146. We suggest that both the 30K superconducting transition is a stable electronic phase and there is an intrinsic hole instability at p=1/8.

4E5.3 Dimensional Crossover in Quasi One-Dimensional and high T_c Superconductors

E. Carlson, University of California Los Angeles, Los Angeles, CA 90095

Presenting Author: E. Carlson

The one-dimensional electron gas exhintis spin-charge separation and power-law spectral responses to many experimentally relevant probes. Ordering in a quasi one-dimensional system is necessarily associated with a dimensional crossover, at which sharp quasiparticle peaks, with small spectral weight, emerge from the incoherent background. Using methods of Abelian bosonization, we derive asymptotically correct expressions for the spectral changes induced by this crossover. Comparison is made with experiments, in the high temperature superconductors, which are electronically quasi one-dimensional on a local scale.

4E5.4 Novel Experiment Elucidating the Stripe-Phase Mechanism in YBCO

T. Venkatesan, Center for Superconductivity Research, University of Maryland, College Park, MD 20742

Presenting Author: T. Venkatesan

Novel experiments are needed to understand the role of striped phases in the transport mechanisms of the cuprate superconductors. While many experiments are continuing to fortify our belief in the striped phase, fast experiments are needed to overcome the dynamics inherent with the striped phases to get meaningful information. Two fast experiments, ion channeling spectrometry and fsec laser induced pair breaking in YBCO show strong evidence for the manifestation of the striped phase. In the former experiment incoherent lattice fluctuations, an inherent property of the striped phase, exhibits unusual temperature dependent cross-overs/transitions, which could be identified with the stripe formation, spin gap opening and the superconducting onset temperatures. In the latter, very sharp optical resonance seen at about 1.5 eV, in the pair breaking, show sub-100 meV structures which would require an inhomogeneous superconducting phase as seen in the striped phase. These experiments off the beaten track may shed new light on the microscopic mechnaism of transport in the cuprate superconductors.

4E5.5 Structural evolution and superconducting phase separation in La₂CuO_4+x

J. Q. Li, L. Chen, and Z. X. Zhao, National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China

Presenting Author: J.Q. Li

The structural evolution of La₂CuO_4+x with both oxygen doping (0<x<0.12) and temperature decreasing (T£ 300K) has been extensively investigated. A phase diagram showing the structural features, the oxygen ordering and the superconducting-phase segregation is presented. A short-range ordering (SRO), assumedly arising from the intercalated oxygen and their resultant structural distortion, is found to be in association with charge-carrier localization. We have interpreted this SRO by a new structural model which gives rise to an intercalated oxygen of x=1/16 and a localized hole concentration of n=1/8. Under an applied magnetic field of H~ 3T, this SRO phenomenon changed evidently.

4E5.6 1/8 Anomaly in the Bi-2212 and Y-123 Phases

Yoji Koike 1, Megumi Akoshima 1, Isao Watanabe 2, and Kanetada Nagamine 2,3. 1 Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan. 2 Muon Science Laboratory, The Institute of Physical and Chemical Research (RIKEN), Wako 351-0198, Japan. 3 Meson Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-MSL), Tsukuba 305-0801, Japan.

Presenting Author: Y. Koike

Experimental work on the so-called 1/8 anomaly in the Bi-2212 and Y-123 phases is reviewed. In the partially Zn-substituted Bi₂Sr₂Ca_1-xY_x(Cu_1-yZn_y)₂O_8+d , we have found anomalous suppression of superconductivity at x=0.30 - 0.35, where the hole concentration per Cu, p, ~1/8. Moreover, it has been revealed from the SR measurements that the Cu-spin fluctuations exhibit slowing-down behavior at low temperatures in these samples. In Y_1-xCa_xBa₂Cu₃O_7-d , the 60K plateau of T_c has been found not to be correlated with the oxygen content but to be interpreted as being due to the suppression of superconductivity at p~1/8. Moreover, the slowing-down behavior of the Cu-spin fluctuations has also been observed in the SR measurements of the partially Zn-substituted YBa₂Cu_3-2yZn_2yO_7-d with p~1/8. These results are suggestive of the stripe correlations of holes and spins tending to be pinned by Zn at p~1/8 in the Bi-2212 and Y-123 phases as well as in the La-214 phase. Accordingly, we conclude that the 1/8 anomaly is common to all high-T_c cuprates with the CuO₂ plane.

4E5.7 Striped Charge/Orbital Ordering in Mixed-Valent Manganites

S-W. Cheong 1,2, C.H. Chen 1, S. Mori 1, M. Uehara 2, K.H. Kim 2, and T.Y. Koo 1. 1 Bell Laboratories, Lucent Technologies, Murray Hill, NJ, 07974, USA. 2 Department of Physics & Astronomy, Rutgers University, Piscataway, NJ, 08854, USA.

Presenting Author: S.-W. Cheong

In doped Mott insulators such as layered cuprates, layered nickelates and perovskite manganites, charge carriers tend to self-organize in such a way as to form stripe patterns at low temperatures. In the case of mixed-valent manganites, orbital degrees of freedom associated with Mn³⁺ can be involved in the striped ordering phenomena. This striped charge/orbital ordering in manganites has been observed in various forms such as long-range commensurate ordering, nano-scale mixtures of two-different commensurate-ordered phases. The way in which the commensurate charge/orbital ordering melts near phase transition temperatures has been also studied in detail. Furthermore, various experimental results also indicate the presence of very-short-range or dynamic correlation of charge/orbital ordering at high temperatures, far above any phase transition temperatures.

The intricate interplay between charge/ordering and ferromagnetic-metallicity plays the essential role in the colossal magnetoresistance (CMR) effects in mixed-valent manganites. In fact, it has been found that the CMR manganites show the sub-m m-scale coexistence of the charge/orbital ordered phase and the ferromagnetic-metallic phase, and that the transport properties are dominated by the percolative conduction through the ferromagnetic-metallic domains. This percolative charge transport can be sensitively influenced by applied magnetic field, causing the CMR effects.